Structural speciation of Oxygen-Containing compounds in fossil fuels using Hydrogen/Deuterium exchange ESI High-Resolution mass spectrometry

Different oxygen species exhibit distinct reactivities and properties in fossil fuels such as petroleum and coal liquid products. Elucidating the structural characteristics of these oxygen-containing compounds is essential for effective process design and optimal feedstock selection. The presented study introduces a method combining hydrogen/deuterium exchange (HDX) with high-resolution mass spectrometry to analyze the molecular composition and functional group types of oxygen-containing compounds. Utilizing perdeuterated methanol and toluene as the solvent, we achieved an average HDX depths exceeding 95% across model compounds. The high degree of HDX allows for the inference of oxygen functional groups based on the number of incorporated deuterium atoms. Application to representative fossil fuels (coal tar, coal liquefaction oil, and crude oil) revealed substantial differences oxygen speciation both between different oils and within individual oils. The HDX provided enhanced structural insights into oxygen-containing compounds in coal-derived liquids. Specifically, phenols in coal tar were found to predominantly contain 1–4 hydroxyl groups, with higher-order hydroxylation being negligible. Oxygenated derivatives with dibenzo-α-pyrone core structures were detected as characteristic components in coal-derived products. In contrast, the predominant dioxygenated species in crude oil were carboxylic acids. Furthermore, HDX analysis elucidated the removal selectivity of oxygen functional groups during hydrotreating of coal liquefaction oil.